Studying the polarization and spectral distortion of the cosmic microwave background (CMB) in tandem with intensity fluctuations of the cosmic infrared background allows us to verify our assumptions on cosmic inflation and investigate the dynamics and evolution of galaxy clusters in the past 10 billion years. Because of its broadband emission and being an all-sky extended source, observing the entire CMB in detail is a very time-consuming and expensive exercise. Fortunately, in the past few years, the on-chip superconducting spectrometer technology has moved out of the lab and into the telescope. With its compact size and background-limited sensitivity, this family of instruments is particularly well-suited for fast and large area observations in a relatively unexplored range of the electromagnetic spectrum. However, recent examples of this technology do not yet reach the requirements needed for large spectroscopic and polarimetric surveys of the CMB. We formulate several of these requirements and introduce novel on-chip components and fabrication techniques. We introduce a crossover to enable distinguishing signal polarization, minimize signal loss by locally optimized lithography of a coplanar waveguide, lower the spectral resolution of microstrip filters by deposition of a dielectric layer, and increase the yield of the spectrometer array by removing individual line shorts. These together have culminated in the successful fabrication of a 14-spaxel integral field unit.

Context. The Deep Spectroscopic HIgh-redshift MApper (DESHIMA) is a broadband integrated superconducting spectrometer for millimeter (mm) and submillimeter (submm) wave astronomy based on kinetic inductance detectors (KIDs). The first generation, DESHIMA 1.0, was successfully tested in 2017. This paper describes the upgrade to DESHIMA 2.0 and presents its characterization in laboratory settings. Aims. We enhanced the performance by increasing the band coverage and efficiency while developing a fast sky-position chopper for the efficient removal of atmospheric fluctuations. Methods. The instrument features NbTiN superconducting microstrip (MS) filters with a low-loss a-SiC:H dielectric and an ultra-wideband leaky-wave antenna. A laboratory setup was designed, incorporating the cryostat housing cryogenic optics and integrated superconducting spectrometer chip comprising 339 KIDs connected to MS filters tuned for (sub)mm wave frequencies. Room-temperature mirrors on a hexapod stage allowed us to precisely position and align the optical elements. The sky-position chopper was positioned on a motor-controlled stage for a fine-tuned control over its position and alignment. The multiplexing capability of KIDs enabled us to simultaneously measure multiple performance metrics across the entire frequency range. Results. DESHIMA 2.0 achieved significant improvements in the performance compared to its predecessor: the measured instantaneous frequency coverage was 200–400 GHz with a mean filter Q_filter of 340 ± 50, the instrument efficiency reached ~8%, indicating a four times wider band coverage and a four times higher sensitivity. The yield rate for the MS filters exceeded 98%. The data loss caused by the sky-position chopper was limited to <20% without beam truncation. The estimated aperture efficiency from measured beam patterns agreed well with the designed value of approximately 70%. The telescope far-field beam patterns calculated from the measured patterns also agree well with the design specifications. We also demonstrated that a new method of absolute frequency calibration is valid using the data from the beam pattern measurement.

We present measurements and simulations of the polarization purity of leaky lens-antenna coupled microwave kinetic inductance detectors (KIDs) at 1.5 THz. From polarized phase and amplitude beam pattern measurements we find the integrated cross-polarization ratio to be at –21.5 dB for 1 f#λ spatial sampling. The measurements agree well with the theoretical description which is based on a combination of in-transmission simulation of the antenna feed, and an in-reception analysis of the antenna-KID system. A neutral density filter limited the power per detector to around 500 fW, enabling these measurements to be taken on detectors that in a low background have a measured noise equivalent power of 5–7×10⁻²⁰ W/√Hz. These combined measurements show that these detectors are excellent candidates for large scale and high-performance imaging polarimetric instruments.

Disordered superconductors offer new impedance regimes for quantum circuits, enable a pathway to protected qubits, and can improve superconducting detectors due to their high kinetic inductance and sheet resistance. The performance of these devices can be limited, however, by quasiparticles—the fundamental excitations of a superconductor. While experiments have shown that disorder affects the relaxation of quasiparticles drastically, the microscopic mechanisms are still not understood. We address this issue by measuring quasiparticle relaxation in a disordered β-Ta film, which we pattern as the inductor of a microwave resonator. We observe that quasiparticle recombination is governed by the phonon scattering time, which is faster than conventional recombination in ordered superconductors. We interpret the results as recombination of localized quasiparticles, induced by disorder, which first delocalize via phonon absorption. We analyze quasiparticle relaxation measurements on superconductors with different degrees of disorder and conclude that this phenomenon is inherent to disordered superconductors.

The presence of quasiparticles typically degrades the performance of superconducting microwave circuits. The readout signal can generate nonequilibrium quasiparticles, which lead to excess microwave loss and decoherence. To understand this effect quantitatively, we measure quasiparticle fluctuations and extract the quasiparticle density across different temperatures, readout powers, and resonator volumes. We find that microwave power generates a higher quasiparticle density as the active resonator volume is reduced and show that this effect sets a sensitivity limit on kinetic inductance detectors. We compare our results with theoretical models of direct microwave photon absorption by quasiparticles and conclude that an unknown, indirect mechanism plays a dominant role in quasiparticle generation. These results provide a route to mitigate quasiparticle generation due to readout power in superconducting devices.

Parallel-plate capacitors (PPC) significantly reduce the size of superconducting microwave resonators, reducing the pixel pitch for arrays of single-photon energy-resolving kinetic inductance detectors (KIDs). The frequency noise of KIDs is typically limited by tunneling two-level systems (TLS), which originate from lattice defects in the dielectric materials required for PPCs. How the frequency noise level depends on the PPC's dimensions has not been experimentally addressed. We measure the frequency noise of 56 resonators with a-SiC:H PPCs, which cover a factor of 44 in PPC area and a factor of 4 in dielectric thickness. To support the noise analysis, we measure the resonators' TLS-induced power-dependent intrinsic loss and temperature-dependent resonance frequency shift. From the TLS models, we expect a geometry-independent microwave loss and resonance frequency shift, which is set by the TLS properties of the dielectric. However, we observe a thickness-dependent microwave loss and resonance frequency shift; this is explained by surface layers that limit the performance of PPC-based resonators. For a uniform dielectric, the frequency noise level should scale directly inversely with the PPC area and thickness. We observe that an increase in PPC size reduces the frequency noise, but the exact scaling is, in some cases, weaker than expected. Finally, we derive engineering guidelines for the design of KIDs based on PPC-based resonators.

Understanding telescope pointing (i.e. line of sight) is important for observing the cosmic microwave background (CMB) and astronomical objects. The Moon is a candidate astronomical source for pointing calibration. Although the visible size of the Moon (30^{`</sup>) is larger than that of the planets, we can frequently observe the Moon once a month with a high signal-to-noise ratio. We developed a method for performing pointing calibration using observational data from the Moon. We considered the tilts of the telescope axes as well as the encoder and collimation offsets for pointing calibration. In addition, we evaluated the effects of the nonuniformity of the brightness temperature of the Moon, which is a dominant systematic error. As a result, we successfully achieved a pointing accuracy of 3.3<sup>`}. This is one order of magnitude smaller than an angular resolution of 36^`. This level of accuracy competes with past achievements in other ground-based CMB experiments using observational data from the planets.

Kinetic inductance detectors (KIDs) are superconducting energy-resolving detectors, sensitive to single photons from the near-infrared to ultraviolet. We study a hybrid KID design consisting of a β-phase tantalum (β-Ta) inductor and a Nb-Ti-N interdigitated capacitor. The devices show an average intrinsic quality factor Qi of 4.3×105±1.3×105. To increase the power captured by the light-sensitive inductor, we 3D print an array of 150×150μm resin microlenses on the backside of the sapphire substrate. The shape deviation between design and printed lenses is smaller than 1μm, and the alignment accuracy of this process is δx=+5.8±0.5μm and δy=+8.3±3.3μm. We measure a resolving power for 1545-402 nm that is limited to 4.9 by saturation in the KID's phase response. We can model the saturation in the phase response with the evolution of the number of quasiparticles generated by a photon event. An alternative coordinate system that has a linear response raises the resolving power to 5.9 at 402 nm. We verify the measured resolving power with a two-line measurement using a laser source and a monochromator. We discuss several improvements that can be made to the devices on a route towards KID arrays with high resolving powers.

Aims. Future actively cooled space-borne observatories for the far-infrared, loosely defined as a 1-10 THz band, can potentially reach a sensitivity limited only by background radiation from the Universe. This will result in an increase in observing speed of many orders of magnitude. A spectroscopic instrument on such an observatory requires large arrays of detectors with a sensitivity expressed as a noise equivalent power NEP = 3 × 10-20 W√ p Hz. Methods. We present the design, fabrication, and characterisation of microwave kinetic inductance detectors (MKIDs) for this frequency range reaching the required sensitivity. The devices are based on thin-film NbTiN resonators which use lens-antenna coupling to a submicron-width aluminium transmission line at the shorted end of the resonator where the radiation is absorbed. We optimised the MKID geometry for a low NEP by using a small aluminium volume of ≈1 μm3 and fabricating the aluminium section on a very thin (100 nm) SiN membrane. Both methods of optimisation also reduce the effect of excess noise by increasing the responsivity of the device, which is further increased by reducing the parasitic geometrical inductance of the resonator. Results. We measure the sensitivity of eight MKIDs with respect to the power absorbed in the detector using a thermal calibration source filtered in a narrow band around 1.5 THz. We obtain a NEPexp(Pabs) = 3:1 ± 0:9 × 10-20 W√ p Hz at a modulation frequency of 200 Hz averaged over all measured MKIDs. The NEP is limited by quasiparticle trapping. Conclusions. The measured sensitivity is sufficient for spectroscopic observations from future, actively cooled space-based observatories. Moreover, the presented device design and assembly can be adapted for frequencies up to ≈10 THz and can be readily implemented in kilopixel arrays.

Typical materials for optical Microwave Kinetic Inductance Detetectors (MKIDs) are metals with a natural absorption of ∼ 30–50% in the visible and near-infrared. To reach high absorption efficiencies (90–100%) the KID must be embedded in an optical stack. We show an optical stack design for a 60 nm TiN film. The optical stack is modeled as sections of transmission lines, where the parameters for each section are related to the optical properties of each layer. We derive the complex permittivity of the TiN film from a spectral ellipsometry measurement. The designed optical stack is optimised for broadband absorption and consists of, from top (illumination side) to bottom: 85 nm SiO₂, 60 nm TiN, 23 nm of SiO₂, and a 100 nm thick Al mirror. We show the modeled absorption and reflection of this stack, which has >80% absorption from 400 to 1550 nm and near-unity absorption for 500–800 nm. We measure transmission and reflection of this stack with a commercial spectrophotometer. The results are in good agreement with the model.

Low-loss deposited dielectrics will benefit superconducting devices such as integrated superconducting spectrometers, superconducting qubits, and kinetic inductance parametric amplifiers. Compared with planar structures, multilayer structures such as microstrips are more compact and eliminate radiation loss at high frequencies. Multilayer structures are most easily fabricated with deposited dielectrics, which typically exhibit higher dielectric loss than crystalline dielectrics. We measure the subkelvin and low-power microwave and millimeter-submillimeter-wave dielectric loss of hydrogenated amorphous silicon carbide (a-SiC:H), using superconducting chips with Nb-Ti-N/a-SiC:H/Nb-Ti-N microstrip resonators. We deposit the a-SiC:H by plasma-enhanced chemical vapor deposition at a substrate temperature of 400°C. The a-SiC:H has a millimeter-submillimeter loss tangent ranging from 0.9×10-4 at 270 GHz to 1.5×10-4 at 385 GHz. The microwave loss tangent is 3.1×10-5. These are the lowest low-power subkelvin loss tangents that have been reported for microstrip resonators at millimeter-submillimeter and microwave frequencies. The a-SiC:H films are free of blisters and have low stress: -20 MPa compressive at 200-nm thickness to 60 MPa tensile at 1000-nm thickness.

Integrated superconducting spectrometer (ISS) technology will enable ultra-wideband, integral-field spectroscopy for (sub)millimeter-wave astronomy, in particular, for uncovering the dust-obscured cosmic star formation and galaxy evolution over cosmic time. Here, we present the development of DESHIMA 2.0, an ISS for ultra-wideband spectroscopy toward high-redshift galaxies. DESHIMA 2.0 is designed to observe the 220–440 GHz band in a single shot, corresponding to a redshift range of z = 3.3–7.6 for the ionized carbon emission ([C II] 158 μ m). The first-light experiment of DESHIMA 1.0, using the 332–377 GHz band, has shown an excellent agreement among the on-sky measurements, the laboratory measurements, and the design. As a successor to DESHIMA 1.0, we plan the commissioning and the scientific observation campaign of DESHIMA 2.0 on the ASTE 10-m telescope in 2023. Ongoing upgrades for the full octave-bandwidth system include the wideband 347-channel chip design and the wideband quasi-optical system. For efficient measurements, we also develop the observation strategy using the mechanical fast sky-position chopper and the sky-noise removal technique based on a novel data-scientific approach. In the paper, we show the recent status of the upgrades and the plans for the scientific observation campaign.

Superconducting resonators and transmission lines are fundamental building blocks of integrated circuits for millimeter-submillimeter astronomy. Accurate simulation of radiation loss from the circuit is crucial for the design of these circuits because radiation loss increases with frequency, and can thereby deteriorate the system performance. Here we show a stratification for a 2.5-dimensional method-of-moment simulator Sonnet EM that enables accurate simulations of the radiative resonant behavior of submillimeter-wave coplanar resonators and straight coplanar waveguides (CPWs). The Sonnet simulation agrees well with the measurement of the transmission through a coplanar resonant filter at 374.6 GHz. Our Sonnet stratification utilizes artificial lossy layers below the lossless substrate to absorb the radiation, and we use co-calibrated internal ports for de-embedding. With this type of stratification, Sonnet can be used to model superconducting millimeter-submillimeter wave circuits even when radiation loss is a potential concern.

Superconducting circuit elements used in millimeter-submillimeter (mm-submm) astronomy would greatly benefit from deposited dielectrics with small dielectric loss and noise. This will enable the use of multilayer circuit elements and thereby increase the efficiency of mm-submm filters and allow for a miniaturization of microwave kinetic inductance detectors (MKIDs). Amorphous dielectrics introduce excess loss and noise compared with their crystalline counterparts, due to two-level system defects of unknown microscopic origin. We deposited hydrogenated amorphous silicon films using plasma-enhanced chemical vapor deposition, at substrate temperatures of 100°C, 250°C, and 350°C. The measured void volume fraction, hydrogen content, microstructure parameter, and bond-Angle disorder are negatively correlated with the substrate temperature. All three films have a loss tangent below 10-5 for a resonator energy of 105 photons, at 120 mK and 4 to 7 GHz. This makes these films promising for MKIDs and on-chip mm-submm filters.

We present a "mix-and-match"process to create large structures with submicrometer features by combining UV contact lithography and 100 kV electron-beam lithography in a single layer of negative-tone resist: Micro-Resist-Technology ma-N1405. The resist is successfully applied for the fabrication of an on-chip terahertz spectrometer, where the design requires 450 nm wide lines and 300 nm wide trenches in a 150 nm thick niobium-titanium-nitride layer, tolerating errors of ± 30 nm. We use a resist thickness of 500 nm, optimized to allow reliable SF 6/O 2-based reactive ion etching of structures with 30 nm accuracy. We find that resist requires an electron-beam cross-linking dose of 1100 μ C / c m 2 for an acceleration voltage of 100 kV in combination with a 180 s 100 °C bake on a hot plate and 45 s development. The smallest resist bars made with our dedicated recipe are 100 nm wide, with the smallest gaps about 300 nm. The difference between the designed and realized feature size is between 2 and 30 nm for structures up to 700 nm wide. The optical exposure dose is 300 m J / c m 2 for the same development time and is optimized to produce a positive sloped edge profile allowing good step coverage for subsequent layers. The resist can be applied, shipped, and processed in a time span of a couple of days without notable deterioration of patterning quality.

We present a lab-on-chip experiment to accurately measure losses of superconducting microstrip lines at microwave and submillimeter wavelengths. The microstrips are fabricated from Nb-Ti-N, which is deposited using reactive magnetron sputtering, and amorphous silicon which is deposited using plasma-enhanced chemical vapor deposition (PECVD). Submillimeter wave losses are measured using on-chip Fabry-Perot resonators (FPRs) operating around 350 GHz. Microwave losses are measured using shunted half-wave resonators with an identical geometry and fabricated on the same chip. We measure a loss tangent of the amorphous silicon at single-photon energies of tan⁡δ=3.7±0.5×10-5 at approximately 6GHz and tan⁡δ=2.1±0.1×10-4 at 350 GHz. These results represent very low losses for deposited dielectrics, but the submillimeter wave losses are significantly higher than the microwave losses, which cannot be understood using the standard two-level system loss model.